Re: "Bust Their Ass

Trabajar duro is correct, as busting your ass is definitely work very hard, but in my opinion you lose "el tono" of the text, which is colloquial. I's say something like "partirse el espinazo" (break your back).

Oh and *partirse el culo (de risa)* (break/bust your ass) in Spain means to have a good laugh!